Linear Technology Analog Devices LT8708 User manual

LT8708
1
Rev 0
For more information www.analog.com
V
BAT2
= 13.5V
I
OUT
= 15A
V
BAT1
(V)
10
12
14
16
94
95
96
97
98
99
100
EFFICIENCY (%)
EFFICIENCY
8708 TA01b
+
–
+
–
CSPOUT
CSNOUT
EXTVCC
VOUTLOMON
FBOUT
INTVCC
GATEVCC
IMON_ON
IMON_OP
ICN
ICP
IMON_INP
IMON_INN
CLKOUTSYNC
SSRT
VCMODE
CSNIN
TG1 BOOST1 SW1 BG1 CSP CSN
LT8708
GND BG2 SW2 BOOST2 TG2
CSPIN
VINCHIP
SHDN
FBIN
VINHIMON
SWEN
LDO33
DIR
RVSOFF
126kHz
FWD (3V)
RVS (0V)
+
+
DB1 DB2
TO
BOOST1
TO
BOOST2
I
LIM
TO DIODE
DB1
LD033
VBAT2
8708 TA01a
I
LIM
10V
TO 16V
BATTERY
VBAT1
10V
TO 16V
BATTERY TO DIODE
DB2
POWER TRANSFER
DECISION LOGIC
TYPICAL APPLICATION
FEATURES DESCRIPTION
80V Synchronous 4-Switch Buck-Boost DC/DC
Controller with Flexible Bidirectional Capability
The LT
®
8708 is a high performance buck-boost switching
regulator controller that operates from an input voltage
that can be above, below or equal to the output voltage.
Features are included to simplify bidirectional power
conversion in battery/capacitor backup systems and other
applications that may need regulation of VOUT, VIN, IOUT,
and/or IIN. Forward and reverse current can be monitored
and limited for the input and output sides of the converter.
Allfourcurrentlimits(forwardinput,reverseinput, forward
output and reverse output) can be set independently using
four resistors on the PCB.
The MODE pin can select between discontinuous conduc-
tion mode (DCM), continuous conduction mode (CCM),
hybridconduction mode(HCM)andBurstMode
®
operation.
Incombination with theDIR (direction) pin, the chip can be
configured to process power only from VIN to VOUT or only
from VOUT to VIN. With a wide 2.8V to 80V input and 1.3V
to 80V output range, the LT8708 is compatible with most
solar, automotive, telecom and battery-powered systems.
12V Bidirectional Dual Battery System with FHCM and RHCM
APPLICATIONS
n Single Inductor Allows VIN Above, Below, or
Equalto VOUT
n Six Independent Forms of Regulation
n VIN Current (Forward and Reverse)
n VOUT Current (Forward and Reverse)
n VIN and VOUT Voltage
n Forward and Reverse Discontinuous Conduction
Mode Supported
n Supports MODE and DIR Pin Changes While Switching
n VINCHIP Range 2.8V (Need EXTVCC > 6.4V) to 80V
n VOUT Range: 1.3V to 80V
n Synchronous Rectification: Up to 99% Efficiency
n Available in 40-Lead (5mm × 8mm) QFN with High
Voltage Pin Spacing
n High Voltage Buck-Boost Converters
n Bidirectional Charging System
n Automotive 48V Systems
All registered trademarks and trademarks are the property of their respective owners.
Efficiency
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LT8708
2
Rev 0
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TABLE OF CONTENTS
Features..................................................... 1
Applications ................................................ 1
Typical Application ........................................ 1
Description.................................................. 1
Absolute Maximum Ratings.............................. 3
Order Information.......................................... 3
Pin Configuration .......................................... 3
Electrical Characteristics ................................. 4
Typical Performance Characteristics ................... 9
Typical Performance Characteristics .................. 11
Pin Functions .............................................. 13
Block Diagram............................................. 15
Operation................................................... 16
Typographical Conventions..................................... 16
Start-Up.................................................................. 16
Start-Up: SHDN Pin ............................................ 16
Start-Up: SWEN Pin............................................ 16
Start-Up: Soft-Start of Switching Regulator....... 17
Control Overview .................................................... 17
Power Switch Control ............................................ 18
Switch Control: Buck Region (VIN >> VOUT)....... 19
Switch Control: Buck-Boost (VIN ≅VOUT)......... 19
Switch Control: Boost Region (VIN << VOUT)......20
Switch Control: Soft-Start ..................................20
Uni and Bidirectional Conduction............................20
Bidirectional Conduction:CCM ..........................21
Unidirectional Conduction:DCM.........................21
Unidirectional Conduction:HCM.........................22
Unidirectional Conduction: Burst Mode ..............22
Error Amplifiers ......................................................22
VOUT Regulation and Sensing .................................23
VOUT:Regulation.................................................23
VOUT:Above Regulation......................................23
VOUT:Below Regulation and Undervoltage .........23
VIN Regulation and Sensing.................................... 24
VIN:Regulation ................................................... 24
VIN:Above Regulation and Overvoltage.............. 24
VIN:Below Regulation......................................... 24
Current Monitoring and Limiting............................. 24
Monitoring and Limiting: IMON Pins .................. 24
Monitoring: ICP and ICN Pins.............................25
INTVCC/EXTVCC/GATEVCC/LDO33 Power ...............25
CLKOUT and Temperature Sensing.........................25
Applications Information ................................ 26
Verify the Power Flow Conditions ...........................26
Operating Frequency Selection ...............................26
Internal Oscillator ...................................................27
SYNC Pin and Clock Synchronization .....................27
CLKOUT Pin and Clock Synchronization .................27
Inductor Current Sensing and Slope Compensation .... 27
RSENSE Selection and Maximum Current ................28
RSENSE Selection: Max RSENSE in the
Boost Region......................................................28
RSENSE Selection: Max RSENSE in the
Buck Region .......................................................30
RSENSE Selection:Final RSENSE Value .................31
RSENSE Filtering ......................................................32
Inductor (L) Selection ............................................32
L Selection: Load Current in Buck and
Boost Regions ....................................................32
L Selection:Subharmonic Oscillations ...............33
L Selection: Maximum Current Rating................33
Power MOSFET Selection .......................................34
CIN and COUT Selection ...........................................37
Schottky Diode (D1, D2, D3, D4) Selection.............38
Topside MOSFET Driver Supply (CB1,DB1,CB2,DB2)... 39
Top Driver: Boost Cap Charge Control Block ......39
Top Driver: Boost Diodes DB1 and DB2 ................... 39
VINHIMON, VOUTLOMON and RVSOFF ..................39
IIN and IOUT Current Monitoring and Limiting .........42
Loop Compensation................................................44
INTVCC Regulators and EXTVCC Connection...........45
LDO33 Regulator ....................................................45
Voltage Lockouts ....................................................46
Junction Temperature Measurement ......................46
Thermal Shutdown ................................................. 47
Efficiency Considerations ...................................... 47
Circuit Board Layout Checklist ............................... 47
Hot Plugging Considerations .................................49
Design Example......................................................49
Typical Applications ...................................... 55
Package Description ..................................... 63
Typical Application ....................................... 64
Related Parts .............................................. 64

LT8708
3
Rev 0
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PIN CONFIGURATIONABSOLUTE MAXIMUM RATINGS
VCSP – VCSN, VCSPIN – VCSNIN,
VCSPOUT – VCSNOUT............................... –0.3V to 0.3V
CSP, CSN Voltage......................................... –0.3V to 3V
VCVoltage (Note 2).................................. –0.3V to 2.2V
RT, FBOUT, SS Voltage ................................ –0.3V to 5V
IMON_INP, IMON_INN, IMON_OP, IMON_ON,
ICP, ICN Voltage....................................... –0.3V to 5V
SYNC Voltage............................................ –0.3V to 5.5V
INTVCC, GATEVCC Voltage............................ –0.3V to 7V
VBOOST1 – VSW1, VBOOST2 – VSW2................ –0.3V to 7V
SWEN, RVSOFF Voltage ............................... –0.3V to 7V
SWEN Current.......................................................0.5mA
RVSOFF Current .......................................................1mA
FBIN, SHDN Voltage .................................. –0.3V to 30V
VINHIMON Voltage..................................... –0.3V to 30V
VOUTLOMON Voltage .................................. –0.3V to 5V
DIR, MODE Voltage...................................... –0.3V to 5V
CSNIN, CSPIN, CSPOUT, CSNOUT Voltage –0.3V to 80V
VINCHIP, EXTVCC Voltage ............................ –0.3V to 80V
SW1, SW2 Voltage ..................................... 81V (Note 6)
BOOST1, BOOST2 Voltage ......................... –0.3V to 87V
BG1, BG2, TG1, TG2 .......................................... (Note 5)
LDO33, CLKOUT ............................................... (Note 8)
Operating Junction Temperature Range
LT8708E (Notes 3, 8)........................ –40°C to 125°C
LT8708I (Notes 3, 8)......................... –40°C to 125°C
LT8708H (Notes 3, 8) ....................... –40°C to 150°C
Storage Temperature Range .................. –65°C to 150°C
(Note 1)
15 16 17 18
TOP VIEW
41
GND
UHG PACKAGE
40-LEAD (5mm ×8mm) PLASTIC QFN
TJMAX = 150°C, θJA = 36°C/W, θJC = 3.8°C/W
EXPOSED PAD (PIN 41) IS GND, MUST BE SOLDERED TO PCB
19 20 21
40 39 38 37 36 35 34
26
27
28
29
30
31
32
33
8
7
6
5
4
3
2
1CLKOUT
SS
SHDN
CSN
CSP
ICN
DIR
FBIN
FBOUT
VC
IMON_INP
IMON_INN
RT
SYNC
CSPIN
CSNIN
CSNOUT
CSPOUT
EXTVCC
ICP
VINHIMON
VOUTLOMON
RVSOFF
BOOST1
TG1
SW1
LDO33 IMON_ON IMON_OP MODE SWEN INTVCC VINCHIP
GND
BG1
GATEVCC
BG2
BOOST2
TG2
SW2
25
24
23
22
9
10
11
12
13
14
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT8708EUHG#PBF LT8708EUHG#TRPBF 8708 40-Lead (5mm × 8mm) Plastic QFN –40°C to 125°C
LT8708IUHG#PBF LT8708IUHG#TRPBF 8708 40-Lead (5mm × 8mm) Plastic QFN –40°C to 125°C
LT8708HUHG#PBF LT8708HUHG#TRPBF 8708 40-Lead (5mm × 8mm) Plastic QFN –40°C to 150°C
Consult ADI Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.

LT8708
4
Rev 0
For more information www.analog.com
ELECTRICAL CHARACTERISTICS
The ldenotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA= 25°C. VINCHIP = 12V, SHDN = 3V, DIR = 3.3V unless otherwise noted (Note 3).
PARAMETER CONDITIONS MIN TYP MAX UNITS
Voltage Supplies and Regulators
VINCHIP Operating Voltage Range EXTVCC = 0V
EXTVCC = 7.5V
l
l
5.5
2.8
80
80
V
V
VINCHIP Quiescent Current Not Switching, VEXTVCC = 0V
SWEN = 3.3V
SWEN = 0V
3.9
2.45
6.5
4.5
mA
mA
VINCHIP Quiescent Current in Shutdown VSHDN = 0V 0 1 μA
EXTVCC Switchover Voltage IINTVCC = –20mA, VEXTVCC Rising l6.15 6.4 6.6 V
EXTVCC Switchover Hysteresis 0.2 V
INTVCC Current Limit Maximum Current Draw from INTVCC and LDO33 Pins
Combined. Regulated from VINCHIP or EXTVCC (12V)
INTVCC = 5.25V
INTVCC = 4.4V
l
l
90
28
127
42
165
55
mA
mA
INTVCC Voltage Regulated from VINCHIP, IINTVCC = 20mA
Regulated from EXTVCC (12V), IINTVCC = 20mA
l
l
6.1
6.1
6.3
6.3
6.5
6.5
V
V
INTVCC Load Regulation IINTVCC = 0mA to 50mA –0.5 –1.5 %
INTVCC, GATEVCC Undervoltage Lockout INTVCC Falling, GATEVCC Connected to INTVCC l4.45 4.65 4.85 V
INTVCC, GATEVCC Undervoltage Lockout Hysteresis GATEVCC Connected to INTVCC 170 mV
INTVCC Regulator Dropout Voltage VINCHIP – VINTVCC, IINTVCC = 20mA 220 mV
LDO33 Pin Voltage 5mA from LDO33 Pin l3.23 3.295 3.35 V
LDO33 Pin Load Regulation ILDO33 = 0.1mA to 5mA –0.25 –1 %
LDO33 Pin Current Limit SYNC = 3V l12 17.25 22 mA
LDO33 Pin Undervoltage Lockout LDO33 Falling 2.96 3.04 3.12 V
LDO33 Pin Undervoltage Lockout Hysteresis 35 mV
Switching Regulator Control
Maximum Current Sense Threshold (VCSP – VCSN) Boost Mode, Minimum M3 Switch Duty Cycle l76 93 110 mV
Maximum Current Sense Threshold (VCSN – VCSP) Buck Mode, Minimum M2 Switch Duty Cycle l68 82 97 mV
Maximum Current Sense Threshold (VCSN – VCSP) Boost Mode, Minimum M3 Switch Duty Cycle l79 93 108 mV
Maximum Current Sense Threshold (VCSP – VCSN) Buck Mode, Minimum M2 Switch Duty Cycle l72 84 96 mV
Gain from VCto Maximum Current Sense Voltage
(VCSP – VCSN) (A5 in the Block Diagram)
Boost Mode
Buck Mode
135
–135
mV/V
mV/V
SHDN Input Voltage High SHDN Rising to Enable the Device l1.175 1.221 1.275 V
SHDN Input Voltage High Hysteresis 40 mV
SHDN Input Voltage Low Device Disabled, Low Quiescent Current
(LT8708E, LT8708I)
(LT8708H)
l
l
0.35
0.3
V
V
SHDN Pin Bias Current VSHDN = 3V
VSHDN = 12V
0
14
1
22
μA
μA
SWEN Rising Threshold Voltage l1.156 1.208 1.256 V
SWEN Threshold Voltage Hysteresis 22 mV
SWEN Output Voltage Low ISWEN = 200μA
SHDN = 0V or VINCHIP = 0V
SHDN = 3V
l
l
0.9
0.2
1.1
0.5
V
V

LT8708
5
Rev 0
For more information www.analog.com
PARAMETER CONDITIONS MIN TYP MAX UNITS
SWEN Internal Pull-Down Release Voltage SHDN = 3V l0.75 0.8 V
MODE Pin Continuous Conduction Mode (CCM)
Threshold
l0.4 V
MODE Pin Hybrid DCM/CCM Mode (HCM) Range l0.8 1.2 V
MODE Pin Discontinuous Conduction Mode (DCM)
Range
l1.6 2.0 V
MODE Pin Burst Mode Operation Threshold l2.4 V
DIR Pin Forward Operation Threshold l1.6 V
DIR Pin Reverse Operation Threshold l1.2 V
RVSOFF Output Voltage Low IRVSOFF = 200μA l0.08 0.5 V
RVSOFF Falling Threshold Voltage l1.155 1.209 1.275 V
RVSOFF Threshold Voltage Hysteresis 165 mV
Soft-Start Charging Current VSS = 0V 13 19 25 μA
ICN Rising Threshold for FDCM Operation MODE = 1V (HCM), DIR = 3.3V l235 255 280 mV
ICN Falling Threshold for CCM Operation MODE = 1V (HCM), DIR = 3.3V l185 205 235 mV
IMON_INP Rising Threshold for RDCM Operation MODE = 1V (HCM), DIR = 0V l235 255 280 mV
IMON_INP Falling Threshold for CCM Operation MODE = 1V (HCM), DIR = 0V l185 205 235 mV
Voltage Regulation Loops (Refer to Block Diagram to Locate Amplifiers)
Regulation Voltage for FBOUT Regulate VCto 1.2V l1.193 1.207 1.222 V
Regulation Voltage for FBIN Regulate VCto 1.2V l1.184 1.205 1.226 V
Line Regulation for FBOUT and FBIN Error Amp
Reference Voltage
VINCHIP = 12V to 80V. Not Switching 0.002 0.005 %/V
FBOUT Pin Bias Current Current Out of Pin 15 nA
FBOUT Error Amp EA4 gm345 μmho
FBOUT Error Amp EA4 Voltage Gain 245 V/V
VOUTLOMON Voltage Activation Threshold Falling l1.185 1.207 1.225 V
VOUTLOMON Threshold Voltage Hysteresis 24 mV
VOUTLOMON Pin Bias Current VVOUTLOMON =1.24V, Current Into Pin
VVOUTLOMON =1.17V, Current Into Pin
l
0.8
0.01
1 1.2
μA
μA
FBIN Pin Bias Current Current Out of Pin 10 nA
FBIN Error Amp EA3 gm235 μmho
FBIN Error Amp EA3 Voltage Gain 150 V/V
VINHIMON Voltage Activation Threshold Rising l1.185 1.207 1.23 V
VINHIMON Threshold Voltage Hysteresis 24 mV
VINHIMON Pin Bias Current VVINHIMON = 1.17V, Current Out of Pin
VVINHIMON = 1.24V, Current Out of Pin l 0.8
0.03
1 1.2
μA
μA
Current Regulation Loops (Refer to Block Diagram to Locate Amplifiers)
Regulation Voltages for IMON_INP and IMON_OP VC= 1.2V l1.185 1.209 1.231 V
Regulation Voltages for IMON_INN and IMON_ON VC =1.2V l1.185 1.21 1.24 V
ELECTRICAL CHARACTERISTICS
The ldenotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA= 25°C. VINCHIP = 12V, SHDN = 3V, DIR = 3.3V unless otherwise noted (Note 3).

LT8708
6
Rev 0
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PARAMETER CONDITIONS MIN TYP MAX UNITS
Line Regulation for IMON_INP, IMON_INN, IMON_OP
and IMON_ON Error Amp Reference Voltage
VINCHIP = 12V to 80V 0.002 0.005 %/V
CSPIN Bias Current VCSPIN = 12V
VCSPIN = 1.5V
0.01
0.01
μA
μA
CSNIN Bias Current BOOST Capacitor Charge Control Block Not Active
VSWEN = 3.3V, VCSPIN = VCSNIN = 12V
VSWEN = 3.3V, VCSPIN = VCSNIN = 1.5V
VSWEN = 0V
84
4.25
0.01
μA
μA
μA
CSPIN, CSNIN Common Mode Operating
Voltage Range
l0 80 V
CSPIN, CSNIN Differential Mode Operating
Voltage Range
l–100 100 mV
IMON_INP Output Current VCSPIN – VCSNIN = 50mV, VCSNIN = 5V
VCSPIN – VCSNIN = 50mV, VCSNIN = 5V
VCSPIN – VCSNIN = 5mV, VCSNIN = 5V
VCSPIN – VCSNIN = 5mV, VCSNIN = 5V
l
l
67
64.5
22.5
20
70
70
25
25
73
75.5
27.5
30
μA
μA
μA
μA
IMON_INN Output Current VCSNIN – VCSPIN = 50mV, VCSNIN = 5V
VCSNIN – VCSPIN = 50mV, VCSNIN = 5V
VCSNIN – VCSPIN = 5mV, VCSNIN = 5V
VCSNIN – VCSPIN = 5mV, VCSNIN = 5V
l
l
66
65
19
18
70
70
25
25
74
75
30.5
32
μA
μA
μA
μA
IMON_INP and IMON_INN Max Output Current l120 μA
IMON_INP Error Amp EA5 gm190 μmho
IMON_INP Error Amp EA5 Voltage Gain 130 V/V
IMON_INN Error Amp EA1 gmFBIN = 0V, FBOUT = 3.3V 190 μmho
IMON_INN Error Amp EA1 Voltage Gain FBIN = 0V, FBOUT = 3.3V 130 V/V
CSPOUT Bias Current VCSPOUT = 12V
VCSPOUT = 1.5V
0.01
0.01
μA
μA
CSNOUT Bias Current BOOST Capacitor Charge Control Block Not Active
VSWEN = 3.3V, VCSPOUT = VCSNOUT = 12V
VSWEN = 3.3V, VCSPOUT = VCSNOUT = 1.5V
VSWEN = 0V
83
4.25
0.01
μA
μA
μA
CSPOUT, CSNOUT Common Mode Operating
Voltage Range
l0 80 V
CSPOUT, CSNOUT Differential Mode Operating
Voltage Range
l–100 100 mV
IMON_OP, ICP Output Current VCSPOUT – VCSNOUT = 50mV, VCSNOUT = 5V
VCSPOUT – VCSNOUT = 50mV, VCSNOUT = 5V
VCSPOUT – VCSNOUT = 5mV, VCSNOUT = 5V
VCSPOUT – VCSNOUT = 5mV, VCSNOUT = 5V
VCSPOUT – VCSNOUT = –5mV, VCSNOUT = 5V
VCSPOUT – VCSNOUT = –5mV, VCSNOUT = 5V
l
l
l
67
65
22.5
20.5
12.5
10.5
70
70
25
25
15
15
73
76
27.5
29
17.5
19.5
μA
μA
μA
μA
μA
μA
IMON_ON, ICN Output Current VCSNOUT – VCSPOUT = 50mV, VCSNOUT = 5V
VCSNOUT – VCSPOUT = 50mV, VCSNOUT = 5V
VCSNOUT – VCSPOUT = 5mV, VCSNOUT = 5V
VCSNOUT – VCSPOUT = 5mV, VCSNOUT = 5V
VCSNOUT – VCSPOUT = –5mV, VCSNOUT = 5V
VCSNOUT – VCSPOUT = –5mV, VCSNOUT = 5V
l
l
l
67
65
22.5
20.5
12.5
10.5
70
70
25
25
15
15
73
75
27.5
29
17.5
19.5
μA
μA
μA
μA
μA
μA
ELECTRICAL CHARACTERISTICS
The ldenotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA= 25°C. VINCHIP = 12V, SHDN = 3V, DIR = 3.3V unless otherwise noted (Note 3).

LT8708
7
Rev 0
For more information www.analog.com
PARAMETER CONDITIONS MIN TYP MAX UNITS
IMON_OP, IMON_ON, ICP and ICN Max
Output Current
l120 μA
IMON_OP Error Amp EA6 gm190 μmho
IMON_OP Error Amp EA6 Voltage Gain 130 V/V
IMON_ON Error Amp EA2 gm FBIN = 0V, FBOUT = 3.3V 190 μmho
IMON_ON Error Amp EA2 Voltage Gain FBIN = 0V, FBOUT = 3.3V 130 V/V
NMOS Gate Drivers
TG1, TG2 Rise Time CLOAD = 3300pF (Note 4) 20 ns
TG1, TG2 Fall Time CLOAD = 3300pF (Note 4) 20 ns
BG1, BG2 Rise Time CLOAD = 3300pF (Note 4) 20 ns
BG1, BG2 Fall Time CLOAD = 3300pF (Note 4) 20 ns
TG1 Off to BG1 On Delay CLOAD = 3300pF Each Driver 90 ns
BG1 Off to TG1 On Delay CLOAD = 3300pF Each Driver 80 ns
TG2 Off to BG2 On Delay CLOAD = 3300pF Each Driver 90 ns
BG2 Off to TG2 On Delay CLOAD = 3300pF Each Driver 80 ns
Minimum On-Time for Main Switch in Boost
Operation (tON(M3,MIN))
Switch M3, CLOAD = 3300pF 200 ns
Minimum On-Time for Synchronous Switch in Buck
Operation (tON(M2,MIN))
Switch M2, CLOAD = 3300pF 200 ns
Minimum Off-Time for Main Switch in Steady-State
Boost Operation
Switch M3, CLOAD = 3300pF 230 ns
Minimum Off-Time for Synchronous Switch in
Steady-State Buck Operation
Switch M2, CLOAD = 3300pF 230 ns
Oscillator
Switch Frequency Range SYNCing or Free Running 100 400 kHz
Switching Frequency, FOSC RT= 365k
RT= 215k
RT= 124k
l
l
l
102
170
310
120
202
350
142
235
400
kHz
kHz
kHz
SYNC High Level for Synchronization l1.3 V
SYNC Low Level for Synchronization l0.5 V
SYNC Clock Pulse Duty Cycle VSYNC = 0V to 2V 20 80 %
Recommended Min SYNC Ratio FSYNC/FOSC 3/4
CLKOUT Output Voltage High VLDO33 – VCLKOUT, 1mA Out of CLKOUT Pin,
ILDO33 = 0μA
100 250 mV
CLKOUT Output Voltage Low 1mA Into CLKOUT Pin 25 100 mV
CLKOUT Duty Cycle TJ= –40°C
TJ= 25°C
TJ= 125°C
22.7
44.1
77
%
%
%
ELECTRICAL CHARACTERISTICS
The ldenotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA= 25°C. VINCHIP = 12V, SHDN = 3V, DIR = 3.3V unless otherwise noted (Note 3).

LT8708
8
Rev 0
For more information www.analog.com
PARAMETER CONDITIONS MIN TYP MAX UNITS
CLKOUT Rise Time CLOAD = 200pF 20 ns
CLKOUT Fall Time CLOAD = 200pF 20 ns
CLKOUT Phase Delay SYNC Rising to CLKOUT Rising, fOSC = 100kHz l160 180 200 degrees
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: Do not force voltage on the VCpin.
Note 3: The LT8708E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LT8708I is guaranteed over the full –40°C to 125°C junction temperature
range. The LT8708H is guaranteed over the full –40°C to 150°C operating
junction temperature range.
Note 4: Rise and fall times are measured using 10% and 90% levels. Delay
times are measured using 50% levels.
Note 5: Do not apply a voltage or current source to these pins. They
mustbe connected to capacitive loads only, otherwise permanent damage
may occur.
Note 6: Negative voltages on the SW1 and SW2 pins are limited, in an
application, by the body diodes of the external NMOS devices, M2 and
M3, or parallel Schottky diodes when present. The SW1 and SW2 pins
are tolerant of these negative voltages in excess of one diode drop below
ground, guaranteed by design.
Note 7: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed the maximum operating junction temperature
when overtemperature protection is active. Continuous operation above
the specified maximum operating junction temperature may impair
devicereliability.
Note 8: Do not force voltage or current into these pins.
ELECTRICAL CHARACTERISTICS
The ldenotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA= 25°C. VINCHIP = 12V, SHDN = 3V, DIR = 3V unless otherwise noted (Note 3).

LT8708
9
Rev 0
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Output Current
(Boost Region – Page 59)
Efficiency vs Output Current
(Buck Region – Page 59)
Efficiency vs Output Current
(Buck–Boost Region – Page 59)
Feedback Voltages Feedback Voltages (Five Parts) Oscillator Frequency
Maximum Inductor Current Sense
Voltage vs Duty Cycle
Inductor Current Sense Voltage at
Minimum Duty Cycle
Maximum Inductor Current Sense
Voltage at Minimum Duty Cycle
LOAD CURRENT (A)
0.01
0.1
1
10
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
8708 G01
HCM
DCM
CCM
V
IN
= 38V
V
OUT
= 47.4V
LOAD CURRENT (A)
0.01
0.1
1
10
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
8708 G02
HCM
DCM
CCM
V
IN
= 51.5V
V
OUT
= 47.4V
LOAD CURRENT (A)
0.01
0.1
1
10
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
8708 G03
HCM
DCM
CCM
V
IN
= 48V
V
OUT
= 47.4V
V
C
= 1.2V
FBOUT
FBIN
IMON_INP
IMON_INN
IMON_ON
IMON_OP
TEMPERATURE (°C)
–45
–20
5
30
55
80
105
130
155
1.17
1.18
1.19
1.20
1.21
1.22
1.23
PIN VOLTAGE (V)
Feedback Voltages
8708 G04
V
C
= 1.2V
TEMPERATURE (°C)
–45
–20
5
30
55
80
105
130
155
1.17
1.18
1.19
1.20
1.21
1.22
1.23
PIN VOLTAGE (V)
8708 G05
R
T
= 124k
R
T
= 215k
R
T
= 365k
TEMPERATURE (°C)
–45
–20
5
30
55
80
105
130
155
0
50
100
150
200
250
300
350
400
FREQUENCY (kHz)
Oscillator Frequency
8708 G06
BUCK REGION
BOOST REGION
M2 OR M3 DUTY CYCLE (%)
0
20
40
60
80
100
0
20
40
60
80
100
120
140
|CSP–CSN| (mV)
8708 G07
BUCK REGION
BOOST REGION
V
C
(V)
0.5
1
1.5
2
–100
–80
–60
–40
–20
0
20
40
60
80
100
–100
–80
–60
–40
–20
0
20
40
60
80
100
CSN–CSP (mV)
CSP–CSN (mV)
8708 G08
BUCK REGION
BOOST REGION
TEMPERATURE (°C)
–45
–20
5
30
55
80
105
130
155
0
20
40
60
80
100
120
|CSP–CSN| (mV)
Voltage at Minimum Duty Cycle
8708 G09

LT8708
10
Rev 0
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Inductor Current Sense
Voltage vs Duty Cycle
INTVCC Line Regulation
(EXTVCC = 0V)
Minimum Inductor Current Sense
Voltage at Minimum Duty Cycle
Maximum and Minimum VCvs SS
INTVCC Line Regulation
(VIN = 12V) IMONx, ICx Output Current
CLKOUT Duty Cycle
LDO33 Pin Regulation
(ILDO33 = 1mA)
VIN Supply Current vs Voltage
(Not Switching)
BUCK REGION
BOOST REGION
M2 OR M3 DUTY CYCLE (%)
0
20
40
60
80
100
–140
–120
–100
–80
–60
–40
–20
0
–|CSP–CSN| (mV)
8708 G10
V
IN
(V)
4
6
8
10
12
14
16
18
20
4.0
4.5
5.0
5.5
6.0
6.5
7.0
INTV
CC
(V)
8708 G11
BUCK REGION
BOOST REGION
TEMPERATURE (°C)
–45
–20
5
30
55
80
105
130
155
–120
–100
–80
–60
–40
–20
0
–|CSP–CSN| (mV)
8708 G12
BUCK REGION
BOOST REGION
MINIMUM V
C
MAXIMUM V
C
T
J
= 25°C
SS (V)
0
0.3
0.6
0.9
1.2
1.5
0
0.5
1.0
1.5
2.0
2.5
V
C
(V)
Maximum and Minimum V
C
vs SS
8708 G13
EXTV
CC
RISING
EXTV
CC
FALLING
EXTV
CC
(V)
4
6
8
10
12
5.5
6.0
6.5
7.0
INTV
CC
(V)
8708 G14
CSPIN–CSNIN (mV)
CSPOUT–CSNOUT (mV)
–200
–100
0
100
200
–25.0
0
25.0
50.0
75.0
100.0
125.0
150.0
175.0
200.0
225.0
250.0
IMONX, ICX CURRENT (µA)
8708 G15
IMON_INN
IMON_ON
ICN
IMON_INP
IMON_OP
ICP
TEMPERATURE (°C)
–45
–20
5
30
55
80
105
130
155
0
20
40
60
80
100
DUTY CYCLE (%)
CLKOUT Duty Cycle
8708 G16
150°C
–45°C
25°C
INTV
CC
(V)
2.5
3
3.5
4
4.5
5
5.5
6
6.5
1.5
2.0
2.5
3.0
3.5
LDO33 (V)
8708 G17
150°C
–45°C
25°C
V
IN
(V)
5
20
35
50
65
80
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
I
IN
(mA)
8708 G18

LT8708
11
Rev 0
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SHDN Pin Current
SHDN and SWEN Pin Thresholds
vs Temperature
Internal VIN UVLO
VINHIMON and VOUTLOMON Pin
Thresholds vs Temperature
VINHIMON and VOUTLOMON Pin
Hysteresis Current vs Temperature
TYPICAL PERFORMANCE CHARACTERISTICS
SHDN
PIN VOLTAGE (V)
0
3
6
9
12
15
18
21
24
27
30
0
2
4
6
8
10
12
14
16
CURRENT INTO PIN (µA)
8708 G19
RISING
FALLING
SHDN
SWEN
TEMPERATURE (°C)
–45
–20
5
30
55
80
105
130
155
1.10
1.12
1.14
1.16
1.18
1.20
1.22
1.24
1.26
1.28
1.30
PIN THRESHOLD VOLTAGE (V)
8708 G20
TEMPERATURE (°C)
–45
–20
5
30
55
80
105
130
155
0
0.5
1.0
1.5
2.0
2.5
3.0
V
IN
UVLO (V)
8708 G21
RISING
FALLING
VINHIMON
VOUTLOMON
TEMPERATURE (°C)
–45
–20
5
30
55
80
105
130
155
1.10
1.12
1.13
1.15
1.16
1.18
1.20
1.21
1.23
1.24
1.26
PIN THRESHOLD VOLTAGE (V)
8708 G22
VINHIMON = 1.24V, OUT OF PIN
VOUTLOMON = 1.17V, INTO PIN
TEMPERATURE (°C)
–45
–20
5
30
55
80
105
130
155
0
0.2
0.4
0.7
0.9
1.1
PIN CURRENT (µA)
8708 G23

LT8708
12
Rev 0
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TYPICAL PERFORMANCE CHARACTERISTICS
Discontinuous Mode (Page 59) Continuous Mode (Page 59)
Continuous Mode (Page 59) Load Step (Page 59)
Load Step (Page 59) Load Step (Page 59)
Continuous Mode (Page 59)
5μs/DIV
8708 G24
IL
5A/DIV
SW1
20V/DIV
SW2
20V/DIV
VBAT = 38V
V
LOAD
= 47.4V
5μs/DIV
8708 G25
IL
5A/DIV
SW1
20V/DIV
SW2
20V/DIV
VBAT = 48V
V
LOAD
= 47.4V
5μs/DIV
8708 G26
IL
5A/DIV
SW1
20V/DIV
SW2
20V/DIV
VBAT = 52V
V
LOAD
= 47.4V
5μs/DIV
8708 G27
IL
5A/DIV
SW1
20V/DIV
SW2
20V/DIV
VBAT = 38V
VLOAD = 47.4V
5ms/DIV
8708 G28
IL
5A/DIV
VLOAD
500mV/DIV
VBAT = 38V
VLOAD = 47.4V
LOAD STEP = 2A TO 4A
5ms/DIV
8708 G29
VLOAD
500mV/DIV
IL
5A/DIV
VBAT = 52V
VLOAD = 47.4V
LOAD STEP = 2A TO 4A
5ms/DIV 8708 G30
VLOAD
500mV/DIV
IL
5A/DIV
VBAT = 48V
VLOAD = 47.4V
LOAD STEP = 2A TO 4A

LT8708
13
Rev 0
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CLKOUT (Pin 1):Clock Output Pin. Use this pin to syn-
chronize one or more compatible switching regulator ICs
to the LT8708. CLKOUT toggles at the same frequency as
the internal oscillator or as the SYNC pin, but is approxi-
mately 180° out of phase. CLKOUT may also be used as a
temperature monitor since the CLKOUT duty cycle varies
linearly with the part’s junction temperature. The CLKOUT
pin can drive capacitive loads up to 200pF.
SS (Pin 2):Soft-Start Pin. Place at least 220nF of capaci-
tance here. Upon start-up, this pin will be charged by an
internal resistor to 3.3V.
SHDN (Pin 3):Shutdown Pin. Tie high to enable chip.
Ground to shut down and reduce quiescent current to a
minimum. Don’t float this pin.
CSN (Pin 4):The (–) Input to the Inductor Current Sense
and DCM Detect Comparator.
CSP (Pin 5):The (+) Input to the Inductor Current Sense
and DCM Detect Comparator. The VCpin voltage and built-
in offsets between CSP and CSN pins, in conjunction with
the RSENSE value, set the inductor current trip threshold.
ICN (Pin 6):Negative VOUT Current Monitor. The current
out of this pin is 20μA plus a current proportional to
the negative average VOUT current. See the Applications
Information section for more information.
DIR (Pin 7):Direction pin when MODE is set for DCM
(discontinuous conduction mode) or HCM (hybrid con-
duction mode) operation. Otherwise this pin is ignored.
Connect the pin to GND to process power from the VOUT
to VIN. Connect the pin to LDO33 to process power from
the VIN to VOUT.
FBIN (Pin 8):VIN Feedback Pin. This pin is connected to
the input of error amplifier EA3 and is used to detect and/
or regulate low VIN voltage.
FBOUT (Pin 9):VOUT Feedback Pin. This pin is connected
to the input of error amplifier EA4 and is used to detect
and/or regulate high VOUT voltage.
VC(Pin 10):Error Amplifier Output Pin. Tie external
compensation network to this pin.
IMON_INP (Pin 11):Positive VIN Current Monitor and
Limit Pin. The current out of this pin is 20μA plus a current
proportionaltothepositive average VIN current. IMON_INP
also connects to error amplifier EA5 and can be used to
limit the maximum positive VIN current. See the Applica-
tions Information section for more information.
IMON_INN (Pin 12):Negative VIN Current Monitor and
Limit Pin. The current out of this pin is 20μA plus a cur-
rent proportional to the negative average VIN current.
IMON_INN also connects to error amplifier EA1 and can
be used to limit the maximum negative VIN current. See
theApplications Information section formore information.
RT (Pin 13):Timing Resistor Pin. Adjusts the switching
frequency. Place a resistor from this pin to ground to set
the frequency. Do not float this pin.
SYNC (Pin 14):To synchronize the switching frequency
to an outside clock, simply drive this pin with a clock. The
high voltage level of the clock needs to exceed 1.3V, and
the low level should be less than 0.5V. Drive this pin to less
than 0.5V to revert to the internal free-running clock. See
theApplications Information section formore information.
BG1, BG2 (Pin 16, Pin 18):Bottom Gate Drive. Drives the
gate of the bottom N-channel MOSFETs between ground
and GATEVCC.
GATEVCC (Pin 17):Power supply for bottom gate drivers.
Must be connected to the INTVCC pin. Do not power from
any other supply. Locally bypass to GND.
BOOST1, BOOST2 (Pin 24, Pin 19):Boosted Floating
Driver Supply. The (+) terminal of the bootstrap capaci-
tor connects here. The BOOST1 pin swings from a diode
voltage below GATEVCC up to VIN + GATEVCC. The BOOST2
pin swings from a diode voltage below GATEVCC up to
VOUT + GATEVCC.
TG1, TG2 (Pin 23, Pin 20):Top Gate Drive. Drives the top
N-channelMOSFETswithvoltageswings equal to GATEVCC
superimposed on the switch node voltages.
SW1, SW2 (Pin 22, Pin 21):Switch Nodes. The (–) ter-
minals of the bootstrap capacitors connect here.
PIN FUNCTIONS

LT8708
14
Rev 0
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RVSOFF (Pin 25):Reverse Conduction Disable Pin. This
is an input/output open-drain pin that requires a pull up
resistor. Pulling this pin low disables reverse current op-
eration. See the Uni and Bidirectional Conduction section
for more information.
VOUTLOMON (Pin 26):VOUT Low Voltage Monitor Pin.
Connect a ±1% resistor divider between VOUT, VOUT-
LOMON and GND to set an undervoltage level on VOUT.
When VOUT is lower than this level, reverse conduction is
disabled to prevent drawing current from VOUT. See the
Applications Information section for more information.
VINHIMON (Pin 27):VIN High Voltage Monitor Pin. Con-
nect a ±1% resistor divider between VIN, VINHIMON and
GND in order to set an overvoltage level on VIN. When VIN
is higher than this level, reverse conduction is disabled
to prevent current flow into VIN. See the Applications
Information section for more information.
ICP (Pin 28):Positive VOUT Current Monitor Pin. The cur-
rent out of this pin is 20μA plus a current proportional to
the positive average VOUT current. See the Applications
Information section for more information.
EXTVCC (Pin 29):External VCC Input. When EXTVCC ex-
ceeds 6.4V (typical), INTVCC will be powered from this
pin. When EXTVCC is lower than 6.4V, the INTVCC will be
powered from VINCHIP.
CSPOUT (Pin 30):The (+) Input to the VOUT Current Moni-
tor Amplifier. This pin and the CSNOUT pin measure the
voltage across the sense resistor, RSENSE2, to provide the
VOUT current signals. Connect this pin to VOUT when not
in use. See Applications Information section for proper
use of this pin.
CSNOUT(Pin31):The(–)InputtotheVOUT CurrentMonitor
Amplifier. Connect this pin to VOUT when not in use. See
Applications Information section for proper use of this pin.
CSNIN (Pin 32):The (–) Input to the VIN Current Monitor
Amplifier. This pin and the CSPIN pin measure the voltage
across the sense resistor, RSENSE1, to provide the VIN cur-
rent signals. Connect this pin to VIN when not in use. See
Applications Information section for proper use of this pin.
CSPIN (Pin 33):The (+) Input to the VIN Current Monitor
Amplifier. Connect this pin to VIN when not in use. See
Applications Information section for proper use of this pin.
VINCHIP (Pin 34):Main Input Supply Pin for the LT8708.
It must be locally bypassed to ground.
INTVCC (Pin 35):6.3VRegulatorOutput.Mustbeconnected
to the GATEVCC pin. INTVCC is powered from EXTVCC when
the EXTVCC voltage is higher than 6.4V, otherwise INTVCC
is powered from VINCHIP. Bypass this pin to ground with
a minimum 4.7μF ceramic capacitor.
SWEN (Pin 36):Switching Regulator Enable Pin. Tie high
through a resistor to enable the switching. Ground to dis-
able switching. This pin is pulled down during shutdown,
a thermal lockout or when an internal UVLO (undervoltage
lockout) is detected. Don’t float this pin. See the Start-Up:
SWEN Pin section for more details.
MODE (Pin 37):Conduction Mode Select Pin. The voltage
applied to this pin sets the conduction mode of the control-
ler. Apply less than 0.4V to enable continuous conduction
mode (CCM). Apply 0.8V to 1.2V to enable the hybrid
conduction mode (HCM). Apply 1.6V to 2.0V to enable
the discontinuous conduction mode (DCM). Apply more
than 2.4V to enable Burst Mode operation.
IMON_OP (Pin 38):Positive VOUT Current Monitor and
Limit Pin. The current out of this pin is 20μA plus a cur-
rent proportional to the positive average VOUT current.
IMON_OP also connects to error amplifier EA6 and can
be used to limit the maximum positive VOUT current. See
theApplications Information section formore information.
IMON_ON (Pin 39):Negative VOUT Current Monitor and
Limit Pin. The current out of this pin is 20μA plus a cur-
rent proportional to the negative average VOUT current.
IMON_ON also connects to error amplifier EA2 and can
be used to limit the maximum negative VOUT current. See
theApplications Information section formore information.
LDO33 (Pin 40):3.3V Regulator Output. Bypass this pin
to ground with a minimum 0.1μF ceramic capacitor.
GND (Pin 15, Exposed Pad Pin 41):Ground. Tie directly
to local ground plane.
PIN FUNCTIONS

LT8708
15
Rev 0
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BLOCK DIAGRAM
Figure 1. Block Diagram
–
+
A5
–
+
A4
–
+
A3
–
+
OSC
–
+
A2
START-UP LOGIC
UV_VIN UV_LDO33 UV_GATEVCC
UV_INTVCC OT
3.3V
LDO
REG
LDO
REG
INTERNAL
SUPPLY2
INTERNAL
SUPPLY1
LDO33
EN
6.3V
LDO
REG
–
+
–
+
SS
RSENSE1
VIN
LDO33
LDO33
–
+
6.3V
LDO
REG
VIN
EXTVCC
INTVCC
EN
1.234V
6.4V
RSHDN2
SHDN
RSHDN1
3.3V
RSENSE
CSN CSP SWEN
VINCHIP
CSNIN
CSPIN
IMON_INN
MODE
CLKOUT
SYNC
RT
IMON_INP
RVS
DIR
RVS
VC
–
+
EA5
–
+
EA6
–
+
EA4
–
+
EA3
1.209V
IMON_INP
–
+
EA1
–
+
EA2
1.21V
IMON_INN
1.207V
1.205V
1.207V
–
+
A6
A1
–
+
–
+
1.207V
–
+
EA7
BOOST CAPACITOR
CHARGE CONTROL
CONTROL
AND
STATE
LOGIC
BOOST1
TG1
SW1
GATEVCC
BG1
GND
BG2
SW2
TG2
BOOST2
RFBIN1
RFBOUT1
RFBOUT2
RFBIN2
FBIN
FBOUT
VIN
RSENSE2
VOUT
RLOMON1
RLOMON2
IMON_ON
ICP
ICN
RLOMON3
RHIMON3
RHIMON2
RHIMON1
IMON_OP
VOUTLOMON
VINHIMON
RVSOFF RRVSOFF
CSNOUT
CSPOUT
CB2
DB2
M3
VIN
LDO33
M4
M2
D3
(OPT)
D4
(OPT)
CB1 M1
D2
(OPT)
D1
(OPT)
DB1
8708 F01

LT8708
16
Rev 0
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OPERATION
TYPOGRAPHICAL CONVENTIONS
The LT8708 is a high performance 4-switch buck-boost
controller that includes features to facilitate bidirectional
current and power flow. Using the LT8708, an application
can command power to be delivered from VIN to VOUT or
from VOUT to VIN as needed. Some terms, listed below, are
usedthroughoutthis data sheet inreferenceto the direction
ofcurrentandpowerflow.Inordertoclarifythesedirection-
based concepts, these terms are defined as follows:
VIN and IIN:The VIN side of circuits drawn in this data
sheet will always be on the left. VIN is con-
nected to the SW1 side of the buck-boost
inductor through M1. IIN is the VIN current.
VOUT and
IOUT:
The VOUT side of circuits drawn in this data
sheet will always be on the right. VOUT is
connected to the SW2 side of the buck-boost
inductorthrough M4.IOUT is theVOUT current.
Supply
(Input):
Power Source. The power source is most
commonly applied to VIN. However, VOUT
can be a Supply (or Input) when power is
being delivered from VOUT to VIN.
Load
(Output):
Devices that are consuming the power. The
Load is most commonly connected to VOUT.
However, VIN can connect to the Load (or
Output) when power is being delivered from
VOUT to VIN.
Forward
Conduction:
Current or power flowing from the VIN or
SW1 node (or side) to the VOUT or SW2 node
(or side) of the circuit. This is generally left
to right on schematics.
Reverse
Conduction:
Current or power flowing from the VOUT or
SW2 node (or side) to the VIN or SW1 node
(or side) of the circuit. This is general right
to left on schematics.
Positive
Current:
Current that flows from the SW1 side of the
buck-boost inductor to the SW2 side. Also
refers to current that flows from VIN and/
or into VOUT.
Reverse
Current:
Current that flows from the SW2 side of the
buck-boost inductor to the SW1 side. Also
refers to current that flows from VOUT and/
or into VIN.
Refer to the Block Diagram (Figure 1) when reading the
following sections about the operation of the LT8708.
START-UP
Figure 2 illustrates the start-up sequence for the LT8708.
Start-Up: SHDN Pin
ThemastershutdownpinforthechipisSHDN.Whendriven
below 0.35V (LT8708E, LT8708I) or 0.3V (LT8708H), the
chip is disabled (CHIP OFF state) and quiescent current is
minimal. Increasing the SHDN voltage can increase qui-
escent current but will not enable the chip until SHDN is
driven above 1.221V (typical) after which the INTVCC and
LDO33 regulators are enabled (SWITCHER OFF 1 state).
External devices powered by LDO33 can become active at
thistimeifenough voltage is available onVINCHIP orEXTVCC
to raise INTVCC, and thus LDO33, to an adequate voltage.
Start-Up: SWEN Pin
The SWEN pin is used to enable the switching regulator
after the chip has also been enabled by driving SHDN high.
SWEN must be pulled high through a resistor to enable
the switching regulator. The typical activation threshold is
1.208V as shown in the Electrical Characteristics section.
When the SWEN pin voltage is below the activation thresh-
old, the CSP-CSN, CSPIN-CSNIN and CSPOUT-CSNOUT
current sense circuits on the chip are disabled.
SWEN has an internal pull-down that is activated when
the switching regulator is unable to operate (see CHIP OFF
and SWITCHER OFF 1 states in Figure 2). After the chip is
able to operate and SWEN is internally pulled down below
0.8V (typical), the internal SWEN pull-down is disabled
and start-up can proceed past the SWITCHER OFF1 state.
LDO33 or INTVCC are convenient nodes to pull SWEN up
to. Choose a pull-up resistor value that limits the current
to less than 200μA when SWEN is pulled low. The SWEN
pin can also be digitally driven through a current limiting
resistor. Note in the Electrical Characteristics section, the
SWEN output low voltage is 0.9V (typical) when SHDN is
low and/or VINCHIP is unpowered. The SWEN output low
is 0.2V when SHDN is 3V and VINCHIP is powered.

LT8708
17
Rev 0
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Figure 2. Start-Up Sequence (All Values are Typical)
OPERATION
SOFT-START
• SS CHARGES UP
• WHEN SS > 0.2V ...
• SWITCHER ENABLED
• VCSOFT-START
• M1, M4 ON-TIME SOFT-START
SWITCHER OFF 2
• SWITCHER DISABLED
• INTVCC AND LDO33 OUTPUTS ENABLED
• SS PULLED LOW
NORMAL MODE
• NORMAL OPERATION
INITIALIZE
SS < 50mV
SS > 1.8V
• SS PULLED LOW
• VCFORCED TO COMMAND NEAR ZERO
CURRENT LIMIT
(INTVCC AND GATEVCC > 4.81V) AND
LDO33 > 3.075V AND SWEN > 1.208V
TJUNCTION < 160°C AND SHDN > 1.221V AND VINCHIP > 2.5V AND
((INTVCC AND GATEVCC < 4.65V)
OR LDO33 < 3.04V)
SWITCHER OFF 1
• SWITCHER DISABLED
• INTVCC AND LDO33 OUTPUTS ENABLED
• SWEN AND SS PULLED LOW
CHIP OFF
SHDN < 1.181V OR
VINCHIP < 2.5V OR
TJUNCTION > 165°C
• SWITCHER OFF
• LDOs OFF
• SWEN PULLED LOW
(INTVCC AND GATEVCC > 4.81V) AND
LDO33 > 3.075V AND SWEN < 0.8V
8708 F02
Start-Up: Soft-Start of Switching Regulator
In the INITIALIZE state, the SS (soft-start) pin is pulled
low to prepare for soft-starting the switching regulator.
After SS has been discharged to less than 50mV, the
SOFT-START state begins. In this state, as SS gradually
rises, the soft-start circuitry provides a gradual ramp of
VCand the inductor current in the appropriate direction
(refer to the VCvs SS Voltage graph in the Typical Per-
formance Characteristics section). This prevents abrupt
surges of inductor current and helps the output voltage
ramp smoothly into regulation. See the Switch Control:
Soft-Start section for information about the power switch
control during soft-start.
During soft-start, an integrated 180k(typical) resistor pulls
SS up to 3.3V. The rising ramp rate of the SS pin voltage
is set by this 180k resistor and the external capacitor
connected to this pin. When SS reaches 1.8V (typical),
the LT8708 exits soft-start and enters normal operation.
Typical values for the external soft-start capacitor range
from 220nF to 2μF. A minimum of 220nF is recommended.
CONTROL OVERVIEW
The LT8708 is a current mode controller that provides
an output voltage above, below or equal to the input volt-
age. It also provides bidirectional current monitoring and
regulation capabilities at both the input and the output.
TheADIproprietarycontrolarchitectureemploysaninductor
current-sensing resistor (RSENSE) in buck, boost or buck-
boostregions ofoperation.Theinductorcurrentiscontrolled
by the voltage on the VCpin, which is the combined output
of six internal error amplifiers EA1–EA6. These amplifiers

LT8708
18
Rev 0
For more information www.analog.com
with constant current (EA6) to a maximum voltage (EA4)
and also reversed, at times, to supply power back to VIN
using the other error amplifiers to regulate VIN and limit
the maximum current.
POWER SWITCH CONTROL
The following discussions about the power switch control
assume that the LT8708 is operating in the continuous
conduction mode (see Bidirectional Conduction:CCM).
Other conduction modes have slight differences that are
discussed later in their respective Conduction sections.
Figure 3 shows a simplified diagram of how the four power
switches are connected to the inductor, VIN, VOUT and
ground. Figure 4 shows the regions of operation for the
LT8708 as a function of VOUT – VIN or switch duty cycle
(DC). The power switches are properly controlled so the
transfer between modes is continuous.
OPERATION
Figure 3. Simplified Diagram of the Buck-Boost Switches
Figure 4. Operating Regions vs VOUT − VIN
TG1
BG1
TG2
BG2
RSENSE
8708 F03
M1
M2
M4
M3
L
SW1 SW2
VIN VOUT
M1 ON, M2 OFF
PWM M3, M4 SWITCHES
M4 ON, M3 OFF
PWM M1, M2 SWITCHES
4-SWITCH PWM
VOUT-VIN
SWITCH
M3 DCMAX
SWITCH
M2 DCMAX
SWITCH
M3 DCMIN
SWITCH
M2 DCMIN
BOOST REGION
BUCK REGION
0BUCK/BOOST REGION
8708 F04
can be used to limit or regulate their respective voltages or
currents as shown in Table 1.
Table 1. Error Amplifiers (EA1 − EA6)
AMPLIFIER NAME PIN NAME USED TO LIMIT OR REGULATE
EA1 IMON_INN Negative IIN
EA2 IMON_ON Negative IOUT
EA3 FBIN VIN Voltage
EA4 FBOUT VOUT Voltage
EA5 IMON_INP Positive IIN
EA6 IMON_OP Positive IOUT
TheVCvoltagetypically has a min-max rangeofabout1.2V.
The maximum VCvoltage commands the most positive
inductor current and, thus, commands the most power
flowfrom VIN to VOUT. Theminimum VCvoltage commands
the most negative inductor current and, thus, commands
the most power flow from VOUT to VIN.
In a simple example of VOUT regulation, the FBOUT pin
receives the VOUT voltage feedback signal which is com-
pared to the internal reference voltage using EA4. Low
VOUT voltage raises VCand, thus, more current flows into
VOUT. Conversely, higher VOUT reduces VC, thus, reducing
the current into VOUT or even drawing current and power
from VOUT.
Note that the current and power flow can also be restricted
to one direction, as needed, by the selected conduction
mode discussed in the Uni and Bidirectional Conduction
section.
As mentioned previously, the LT8708 also provides
bidirectional current regulation capabilities at both the
input and the output. The VOUT current can be regulated
or limited in the forward and reverse directions (EA6 and
EA2, respectively). The VIN current can also be regulated
or limited in the forward direction and reverse directions
(EA5 and EA1, respectively).
In a common application, VOUT might be regulated using
EA4,whiletheremainingerroramplifiersaremonitoring for
excessive input or output current or an input undervoltage
condition. In other applications, such as a battery backup
system, a battery connected to VOUT might be charged

LT8708
19
Rev 0
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Switch Control: Buck Region (VIN >> VOUT)
When VIN is significantly higher than VOUT, the part will run
in the buck region. In this region M3 is always off and switch
M4 is always on. At the start of every cycle, synchronous
switch M2 is turned on first. Inductor current is sensed by
amplifier A4 while switch M2 is on. A slope compensation
ramp isadded to the sensedvoltage which is thencompared
by A5 to a reference that is proportional to VC. After the
sensed inductorcurrent fallsbelow the reference,switch M2
is turned off and switch M1 is turned on for the remainder
of the cycle. Switches M1 and M2 will alternate, behaving
like a typical synchronous buck regulator. Figure 5 shows
the switching waveforms in the buck region.
When VIN is much higher than VOUT, the duty cycle of
switch M2 will increase, causing the M2 switch off-time
to decrease. The M2 switch off-time should be kept above
230ns (typical, see Electrical Characteristics) to maintain
steady-stateoperation and avoid dutycyclejitter, increased
output ripple and reduction in maximum output current.
Switch Control: Buck-Boost (VIN ≅VOUT)
When VIN is close to VOUT, the controller operates in the
buck-boost region. Figure 6 shows typical waveforms
in this region. Every cycle, if the controller starts with
switches M2 and M4 turned on, the controller first oper-
ates as if in the buck region. When A5 trips, switch M2
is turned off and M1 is turned on until the middle of the
clock cycle. Next, switch M4 turns off and M3 turns on.
The LT8708 then operates as if in boost mode until A2
trips. Finally, switch M3 turns off and M4 turns on until
the end of the cycle.
If the controller starts with switches M1 and M3 turned
on, the controller first operates as if in the boost region.
When A2 trips, switch M3 is turned off and M4 is turned
on until the middle of the clock cycle. Next, switch M1
Figure 5. Buck Region (VIN >> VOUT)
OPERATION
SWITCH M1
CLOCK
SWITCH M2
SWITCH M3
SWITCH M4
IL
OFF
ON
8708 F05
The part will continue operating in the buck region over a
range of switch M2 duty cycles. The duty cycle of switch
M2 in the buck region is given by:
DC(M2,BUCK) =1– VOUT
V
IN
⎛
⎝
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟•100%
As VIN and VOUT get closer to each other, the duty cycle
decreases until the minimum duty cycle of the converter,
in the buck region, reaches DC(ABSMIN,M2,BUCK). If the duty
cycle becomes lower than DC(ABSMIN,M2,BUCK) the part will
move to the buck-boost region.
DC(ABSMIN,M2,BUCK) ≅tON(M2,MIN) • ƒ • 100%
where:
tON(M2,MIN) is the minimum on-time for the synchro-
nous switch in buck operation (200ns typical, see
Electrical Characteristics).
ƒ is the switching frequency. Figure 6. Buck-Boost Region
SWITCH M1
CLOCK
SWITCH M2
SWITCH M3
SWITCH M4
IL
8708 F06a
6(a) Buck-Boost Region (VIN ≥ VOUT)
SWITCH M1
CLOCK
SWITCH M2
SWITCH M3
SWITCH M4
IL
8708 F06b
6(a) Buck-Boost Region (V
IN
≤ V
OUT
)

LT8708
20
Rev 0
For more information www.analog.com
turns off and M2 turns on. The LT8708 then operates as
if in buck mode until A5 trips. Finally, switch M2 turns off
and M1 turns on until the end of the cycle.
Switch Control: Boost Region (VIN << VOUT)
When VOUT is significantly higher than VIN, the part oper-
ates in the boost region. In this region switch M1 is always
on and switch M2 is always off. At the start of every cycle,
switch M3 is turned on first. Inductor current is sensed by
amplifier A4 while switch M3 is on. A slope compensation
ramp isadded to the sensedvoltage which is thencompared
(A2) to a reference that is proportional to VC. After the
sensed inductor current rises above the reference voltage,
switch M3 is turned off and switch M4 is turned on for the
remainder of the cycle. Switches M3 and M4 will alternate,
behaving like a typical synchronous boost regulator.
The part will continue operating in the boost region over a
range of switch M3 duty cycles. The duty cycle of switch
M3 in the boost region is given by:
DC(M3,BOOST) =1– VIN
V
OUT
⎛
⎝
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟•100%
As VIN and VOUT get closer to each other, the duty cycle
decreases until the minimum duty cycle of the converter,
in the boost region, reaches DC(ABSMIN,M3,BOOST). If the
duty cycle becomes lower than DC(ABSMIN,M3,BOOST), the
part will move to the buck-boost region.
DC(ABSMIN,M3,BOOST) ≅tON(M3,MIN) • ƒ • 100%
where:
tON(M3,MIN) is the minimum on-time for the main
switchin boost operation(200ns typical,see Electrical
Characteristics).
ƒ is the switching frequency.
When VOUT is much higher than VIN, the duty cycle of
switch M3 will increase, causing the M3 switch off-time
to decrease. The M3 switch off-time should be kept above
230ns (typical, see Electrical Characteristics) to maintain
steady-stateoperation and avoid dutycyclejitter, increased
output ripple and reduction in maximum output current.
Figure 7. Boost Region (VIN << VOUT)
Switch Control: Soft-Start
During soft-start, the LT8708 operates in the same three
regions discussed above (buck, buck-boost and boost).
However, a few differences in switch control happen dur-
ing soft-start.
First, M1 and M4 are not turned on simultaneously while
SS ramps up to 0.8V (typical). When M1 and M4 would
normally both be on, they are instead turned off, leaving
all four switches off. After SS rises above 0.8V, during the
time when M1 and M4 would normally both be on, they
are turned on briefly instead. This brief amount of time
increases as SS rises until M1 & M4 are allowed to remain
on as long as the normal switching sequence requires.
Second, M2 and M3 will occasionally turn on together for
one cycle to refresh both boost capacitors. This refresh
cyclehappens because M1 and M4 switch more frequently
during soft-start than in normal operation. As such, the
Boost Capacitor Charge Control block (see Figure 1) can-
not always keep the boost capacitors charged. M2 and M3
are turned on when either BOOSTx-SWx voltage drops
below 5V (typical). Note that during the refresh cycle,
the inductor current slope is nearly zero, thus the boost
capacitors can be refreshed without much disturbance to
the ongoing switching operations.
UNI AND BIDIRECTIONAL CONDUCTION
The LT8708 has one bidirectional and three unidirectional
currentconductionmodes,primarily selected by the MODE
pin. The bidirectional mode (CCM:continuous conduction
mode) allows current and power to flow from VIN to VOUT,
orvice versa, under controlofthe VCpin. The unidirectional
OPERATION
SWITCH M1
CLOCK
SWITCH M2
SWITCH M3
SWITCH M4
IL
OFF
ON
8708 F07